X-mimo systems with multi-transmitters and multi-receivers

ABSTRACT

A method and apparatus for transmitting and receiving a wireless transmission of a plurality of data streams in a wireless communication system having a plurality of nodes is disclosed. Each node has multiple antennas. The method involves receiving first and second data streams from respective first and second nodes at a receiver node, causing the receiver node to generate a receive filter for decoding each of the received data streams, and causing the receiver node to transmit receive filter information for each of the first and second data streams, the receive filter information facilitating precoding of the first and second data streams for simultaneous transmission within a common frequency band to the receiver node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication 61/100,118, filed on Sep. 25, 2008.

MICROFICHE APPENDIX

Not applicable.

TECHNICAL FIELD

This application relates to wireless communication techniques ingeneral, and to a techniques of the present disclosure, in particular.

SUMMARY

Aspects and features of the present application will become apparent tothose ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the disclosure in conjunctionwith the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present application will now be described, by way ofexample only, with reference to the accompanying drawing figures,wherein:

FIG. 1 is a block diagram of a cellular communication system;

FIG. 2 is a block diagram of an example base station that might be usedto implement some embodiments of the present 5 application;

FIG. 3 is a block diagram of an example wireless terminal that might beused to implement some embodiments of the present application;

FIG. 4 is a block diagram of an example relay station that might be usedto implement some embodiments of the present application;

FIG. 5 is a block diagram of a logical breakdown of an example OFDMtransmitter architecture that might be used to implement someembodiments of the present application;

FIG. 6 is a block diagram of a logical breakdown of an example OFDMreceiver architecture that might be used to implement some embodimentsof the present application;

FIG. 7( a) is an example SC-FDMA transmitter for single-in single-out(SISO) configuration provided in accordance with one embodiment of thepresent application;

FIG. 7( b) is an example SC-FDMA receiver for SISO configurationprovided in accordance with the embodiment of FIG. 7( a);

FIG. 8 illustrates NB-NB cooperation;

FIG. 9 illustrates NB-relay cooperation;

FIG. 10 illustrates relay-relay cooperation;

FIG. 11 illustrates a multi-access channel or uplink system;

FIG. 12 illustrates a broadcast channel or down-link system;

FIG. 13 illustrates an interference channel or concurrent point-to-pointcommunication system;

FIG. 14 illustrates a proposed scheme;

FIG. 15 illustrates a basic configuration for two transmitters and tworeceivers;

FIG. 16 illustrates an application of the proposed scheme in MIMOdownlink with parallel relays;

FIG. 17 illustrates an application of the proposed scheme in MIMO uplinkwith parallel relays;

FIG. 18 illustrates an application of the proposed scheme in MIMOinterference channels with parallel relays;

FIG. 19 illustrates an example for signaling scheme for the proposedscenario based on zero-forcing scheme;

FIG. 20 illustrates multipoint-to-point and point-to-multipointcommunication;

FIG. 21 illustrates interference channel or concurrent point-to-pointand X-MIMO;

FIG. 22 illustrates an application in downlink with parallel relays;

FIG. 23 illustrates an application in uplink with parallel relays; and

FIG. 24 illustrates an application in interference channels withparallel relays.

Like reference numerals are used in different figures to denote similarelements.

DETAILED DESCRIPTION OF THE DRAWINGS Wireless System Overview

Referring to the drawings, FIG. 1 shows a base station controller (BSC)10 which controls wireless communications within multiple cells 12,which cells are served by corresponding base stations (BS) 14. In someconfigurations, each cell is further divided into multiple sectors 13 orzones (not shown). In general, each base station 14 facilitatescommunications using OFDM with mobile and/or wireless terminals 16,which are within the cell 12 associated with the corresponding basestation 14. The movement of the mobile terminals 16 in relation to thebase stations 14 results in significant fluctuation in channelconditions. As illustrated, the base stations 14 and mobile terminals 16may include multiple antennas to provide spatial diversity forcommunications. In some configurations, relay stations 15 may assist incommunications between base stations 14 and wireless terminals 16.Wireless terminals 16 can be handed off 18 from any cell 12, sector 13,zone (not shown), base station 14 or relay 15 to an other cell 12,sector 13, zone (not shown), base station 14 or relay 15. In someconfigurations, base stations 14 communicate with each and with anothernetwork (such as a core network or the internet, both not shown) over abackhaul network 11. In some configurations, a base station controller10 is not needed.

With reference to FIG. 2, an example of a base station 14 isillustrated. The base station 14 generally includes a control system 20,a baseband processor 22, transmit circuitry 24, receive circuitry 26,multiple antennas 28, and a network interface 30. The receive circuitry26 receives radio frequency signals bearing information from one or moreremote transmitters provided by mobile terminals 16 (illustrated in FIG.3) and relay stations 15 (illustrated in FIG. 4). A low noise amplifierand a filter (not shown) may cooperate to amplify and remove broadbandinterference from the signal for processing. Downconversion anddigitization circuitry (not shown) will then downconvert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14, either directly or with the assistance of a relay15.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by one or more carrier signalshaving a desired transmit frequency or frequencies. A power amplifier(not shown) will amplify the modulated carrier signals to a levelappropriate for transmission, and deliver the modulated carrier signalsto the antennas 28 through a matching network (not shown). Modulationand processing details are described in greater detail below.

With reference to FIG. 3, an example of a mobile terminal 16 isillustrated. Similarly to the base station 14, the mobile terminal 16will include a control system 32, a baseband processor 34, transmitcircuitry 36, receive circuitry 38, multiple antennas 40, and userinterface circuitry 42. The receive circuitry 38 receives radiofrequency signals bearing information from one or more base stations 14and relays 15. A low noise amplifier and a filter (not shown) maycooperate to amplify and remove broadband interference from the signalfor processing. Downconversion and digitization circuitry (not shown)will then downconvert the filtered, received signal to an intermediateor baseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. The baseband processor 34 is generallyimplemented in one or more digital signal processors (DSPs) andapplication specific integrated circuits (ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, video, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate one or more carrier signals that is at a desired transmitfrequency or frequencies. A power amplifier (not shown) will amplify themodulated carrier signals to a level appropriate for transmission, anddeliver the modulated carrier signal to the antennas 40 through amatching network (not shown). Various modulation and processingtechniques available to those skilled in the art are used for signaltransmission between the mobile terminal and the base station, eitherdirectly or via the relay station.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation utilizes the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalrecovers the transmitted information. In practice, the IFFT and FFT areprovided by digital signal processing carrying out an Inverse DiscreteFourier Transform (IDFT) and Discrete Fourier Transform (DFT),respectively. Accordingly, the characterizing feature of OFDM modulationis that orthogonal carrier waves are generated for multiple bands withina transmission channel. The modulated signals are digital signals havinga relatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

In operation, OFDM is preferably used for at least downlink transmissionfrom the base stations 14 to the mobile terminals 16. Each base station14 is equipped with “n” transmit antennas 28 (n>=1), and each mobileterminal 16 is equipped with “m” receive antennas 40 (m>=1). Notably,the respective antennas can be used for reception and transmission usingappropriate duplexers or switches and are so labeled only for clarity.

When relay stations 15 are used, OFDM is preferably used for downlinktransmission from the base stations 14 to the relays 15 and from relaystations 15 to the mobile terminals 16.

With reference to FIG. 4, an example of a relay station 15 isillustrated. Similarly to the base station 14, and the mobile terminal16, the relay station 15 will include a control system 132, a basebandprocessor 134, transmit circuitry 136, receive circuitry 138, multipleantennas 130, and relay circuitry 142. The relay circuitry 142 enablesthe relay 14 to assist in communications between a base station 16 andmobile terminals 16. The receive circuitry 138 receives radio frequencysignals bearing information from one or more base stations 14 and mobileterminals 16. A low noise amplifier and a filter (not shown) maycooperate to amplify and remove broadband interference from the signalfor processing. Downconversion and digitization circuitry (not shown)will then downconvert the filtered, received signal to an intermediateor baseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 134 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. The baseband processor 134 is generallyimplemented in one or more digital signal processors (DSPs) andapplication specific integrated circuits (ASICs).

For transmission, the baseband processor 134 receives digitized data,which may represent voice, video, data, or control information, from thecontrol system 132, which it encodes for transmission. The encoded datais output to the transmit circuitry 136, where it is used by a modulatorto modulate one or more carrier signals that is at a desired transmitfrequency or frequencies. A power amplifier (not shown) will amplify themodulated carrier signals to a level appropriate for transmission, anddeliver the modulated carrier signal to the antennas 130 through amatching network (not shown). Various modulation and processingtechniques available to those skilled in the art are used for signaltransmission between the mobile terminal and the base station, eitherdirectly or indirectly via a relay station, as described above.

With reference to FIG. 5, a logical OFDM transmission architecture willbe described. Initially, the base station controller 10 will send datato be transmitted to various mobile terminals 16 to the base station 14,either directly or with the assistance of a relay station 15. The basestation 14 may use the channel quality indicators (CQIs) associated withthe mobile terminals to schedule the data for transmission as well asselect appropriate coding and modulation for transmitting the scheduleddata. The CQIs may be directly from the mobile terminals 16 ordetermined at the base station 14 based on information provided by themobile terminals 16. In either case, the CQI for each mobile terminal 16is a function of the degree to which the channel amplitude (or response)varies across the OFDM frequency band.

Scheduled data 44, which is a stream of bits, is scrambled in a mannerreducing the peak-to-average power ratio associated with the data usingdata scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. Again, thechannel coding for a particular mobile terminal 16 is based on the CQI.In some implementations, the channel encoder logic 50 uses known Turboencoding techniques. The encoded data is then processed by rate matchinglogic 52 to compensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably,Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key(QPSK) modulation is used. The degree of modulation is preferably chosenbased on the CQI for the particular mobile terminal. The symbols may besystematically reordered to further bolster the immunity of thetransmitted signal to periodic data loss caused by frequency selectivefading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. When spatialdiversity is desired, blocks of symbols are then processed by space-timeblock code (STC) encoder logic 60, which modifies the symbols in afashion making the transmitted signals more resistant to interferenceand more readily decoded at a mobile terminal 16. The STC encoder logic60 will process the incoming symbols and provide “n” outputscorresponding to the number of transmit antennas 28 for the base station14. The control system 20 and/or baseband processor 22 as describedabove with respect to FIG. 5 will provide a mapping control signal tocontrol STC encoding. At this point, assume the symbols for the “n”outputs are representative of the data to be transmitted and capable ofbeing recovered by the mobile terminal 16.

For the present example, assume the base station 14 has two antennas 28(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing, alone or in combination with otherprocessing described herein. The IFFT processors 62 will preferablyoperate on the respective symbols to provide an inverse FourierTransform. The output of the IFFT processors 62 provides symbols in thetime domain. The time domain symbols are grouped into frames, which areassociated with a prefix by prefix insertion logic 64. Each of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, pilotsignals known by the intended mobile terminal 16 are scattered among thesub-carriers. The mobile terminal 16, which is discussed in detailbelow, will use the pilot signals for channel estimation.

Reference is now made to FIG. 6 to illustrate reception of thetransmitted signals by a mobile terminal 16, either directly from basestation 14 or with the assistance of relay 15. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry72 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time domain to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionlogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency. Continuingwith FIG. 6, the processing logic compares the received pilot symbolswith the pilot symbols that are expected in certain sub-carriers atcertain times to determine a channel response for the sub-carriers inwhich pilot symbols were transmitted. The results are interpolated toestimate a channel response for most, if not all, of the remainingsub-carriers for which pilot symbols were not provided. The actual andinterpolated channel responses are used to estimate an overall channelresponse, which includes the channel responses for most, if not all, ofthe sub-carriers in the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols.

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

In parallel to recovering the data 116, a CQI, or at least informationsufficient to create a CQI at the base station 14, is determined andtransmitted to the base station 14. As noted above, the CQI may be afunction of the carrier-to-interference ratio (CR), as well as thedegree to which the channel response varies across the varioussub-carriers in the OFDM frequency band. For this embodiment, thechannel gain for each sub-carrier in the OFDM frequency band being usedto transmit information is compared relative to one another to determinethe degree to which the channel gain varies across the OFDM frequencyband. Although numerous techniques are available to measure the degreeof variation, one technique is to calculate the standard deviation ofthe channel gain for each sub-carrier throughout the OFDM frequency bandbeing used to transmit data.

Referring to FIG. 7, an example SC-FDMA transmitter 7(a) and receiver7(b) for single-in single-out (SISO) configuration is illustratedprovided in accordance with one embodiment of the present application.In SISO, mobile stations transmit on one antenna and base stationsand/or relay stations receive on one antenna. FIG. 7 illustrates thebasic signal processing steps needed at the transmitter and receiver forthe LTE SC-FDMA uplink. In some embodiments, SC-FDMA (Single-CarrierFrequency Division Multiple Access) is used. SC-FDMA is a modulation andmultiple access scheme introduced for the uplink of 3GPP Long TermEvolution (LTE) broadband wireless fourth generation (4G) air interfacestandards, and the like. SC-FDMA can be viewed as a DFT pre-coded OFDMAscheme, or, it can be viewed as a single carrier (SC) multiple accessscheme. There are several similarities in the overall transceiverprocessing of SC-FDMA and OFDMA. Those common aspects between OFDMA andSC-FDMA are illustrated in the OFDMA TRANSMIT CIRCUITRY and OFDMARECEIVE CIRCUITRY, as they would be obvious to a person having ordinaryskill in the art in view of the present specification. SC-FDMA isdistinctly different from OFDMA because of the DFT pre-coding of themodulated symbols, and the corresponding IDFT of the demodulatedsymbols. Because of this pre-coding, the SC-FDMA sub-carriers are notindependently modulated as in the case of the OFDMA sub-carriers. As aresult, PAPR of SC-FDMA signal is lower than the PAPR of OFDMA signal.Lower PAPR greatly benefits the mobile terminal in terms of transmitpower efficiency.

FIGS. 1 to 7 provide one specific example of a communication system thatcould be used to implement embodiments of the application. It is to beunderstood that embodiments of the application can be implemented withcommunications systems having architectures that are different than thespecific example, but that operate in a manner consistent with theimplementation of the embodiments as described herein.

Further details of embodiments of aspects of the present application areprovided below.

The above-described and below-described embodiments of the presentapplication are intended to be examples only. Those of skill in the artmay effect alterations, modifications and variations to the particularembodiments without departing from the scope of the application.

Keywords for Searching:

-   MIMO-   Multiple transmit and multiple receive antennas-   Multiple transmitters and multiple receivers-   Spatial multiplexing-   Multi-user MIMO-   Non-cooperative communications-   Spectrum Sharing-   Parallel relaying-   Interference management-   Network coding-   Sofr handover-   Mesh networking-   Self-organized networks-   WiFi-   GiFi-   Gigabit MIMO    Products that will use this Application:-   Potentially extendable to WiMAX and LTE-   IEEE802.16m-   1EEE802.11n-   1EEE802.11VHT-   IEEE802.15-   Beyond 4G systems-   IMT-advanced systems-   Is this Application relevant to a Standards Activity? yes-   If so, give details:-   Plan to develop the next generation standard based on this concept-   Especially IEEE802.16m and LTE-Advanced

Technical Information Brief Description of the Application:

This Application configures more than one multi-antenna transmitter andmore than one multi-antenna-receiver, each transmitter has the knowledgeof the MIMO channels information and there is NO data-exchangingrequired between the transmitters (non cooperative transmission), thereis NO data-exchanging in the receiver side. Linear pre-coding can beapplied at transmitters and/or receivers. The transmit and receiveper-coding/filtering is performed such that the dimension of theinterference is minimized. This is core value of this Application, sincewe can minimized the number of transmit/receive antennas to achievehighest spectral efficiency, as an example, if we have 2 users each with2 transmits, in the conventional receiver requires 4 receive antennasfor each user in order to achieve multiplexing again of 4, with thisApplication, we only need 3 receive antennas for each user to achievethe same multiplexing again of 4 without penalty on the transmit powerand bandwidth, this architecture is called X-MIMO. The basic scheme ofX-MIMO can be generalized to many wireless/wireline appellation, such asmulti-hop relay, total distributed MIMO networking.

Problem Solved by the Application:

This Application provides solutions for the following fundamentaldifficulties in distributed broadband wireless networking: e.g. (1)achieving the higher multiplexing gain without exchange the data at bothtransmitter side and receiver side, which no prior arts can do this,this is a major obstacle to enable disturbed multi-user communications(2) this scheme enable the relay node sharing between the multiple datapath to support distinct source-destination routing, for example in MIMOdownlink system, where more than one relay nodes and more than onereceivers (3) to achieving a give multiplexing gain, this solutionrequires minimum number of transmit/receive antennas or (4) with thegiven number of transmit/receive antenna, this scheme achieve themaximum multiplexing gain.

Solutions that have been tried and why they didn't Work:

Two high cost alternative solutions to achieve the same performance are(1) Using an additional backbone system to connect transmitters orreceivers which enable us to apply advance schemes such as dirty paperprecoding (2) Using more than transmit/receive antennas.

For the first alternative, in many practical case the connectionsbetween transmits or receivers are not possible, for the secondalternative, additional more antennas will be limited by the device formfactor, both solutions are expensive.

Specific Elements or Steps that Solved the Problem and how they do it:

The basic elements of this Application are several multi-antennatransmitters and several multi-antenna receivers and associated pilots.The operational steps are the followings:

[1] Each transmitter sends the pilot for each antenna and the pilot foreach transmitter is orthogonal.

[2] Each receiver estimates all the incoming MIMO channels and computethe specific receive filters and each receiver feeds back the compoundfilter and MIMO channel to via dedicated feedback channel to a specifictransmitter

[3] Each transmitter computes the linear pre-coding filter based on suchfeedback information from the receiver

[4] Each transmitter sends the pre-coded data

[5] Each receiver demodulates the corresponding data from filteredreceive signal

Commercial Value of the Application to Nortel and Nortel's MajorCompetitors:

This Application can be used as Nortel-specific proprietaryimplementation or it can be standardized in the next generationbroadband wireless standards.

3GPP TSG-Ran Working Group 1 Meeting #54b R1-083870 Prague, CzechRepublic, Sep. 29^(th)-Oct. 3^(rd), 2008 Agenda Item 11 Source NortelTitle LTE-A Downlink Multi-site MIMO Cooperation Document for Discussion

1 Introduction

Cooperation between neighbouring sites in a LTE-A system improvescoverage for the cell edge users as well as total cell throughput. Inthe LTE standard, such cooperation is limited and does not involvescheduling, data sharing or channel state information state exchangebetween the transmitters. There are several proposals to adoptmulti-site cooperation techniques in the LTE-A standard [1-4]. In thiscontribution, we study different cooperation scenarios and propose somesolutions for further study for possible exploiting in the LTE-Astandard.

2 Cooperation Scenarios

Different system setups allow for different cooperation level. Datasharing, CSI sharing and antenna configuration are among the aspects forconsideration in multi-site cooperation. Here, we study some aspects ofthe system that need to be considered for each cooperation technique.

The cooperating sites may be located to the same cell or located indifferent cells. With multi-cell cooperation as shown in FIG. 8, allparticipating sites have access to backhaul and hence allowing for dataexchange and CSI exchange. However, this requires a distributedscheduling mechanism that enables cooperation for such cell edge users.See FIG. 8.

Same-site cooperation includes NB-relay and relay-relay cooperation aswell as distributed antenna setups. In these cases, a centralizedscheduler is possible. However, for the case of relays, a mechanism toshare data and CSI between the nodes is open for study. See FIGS. 9 and10.

Antenna setup at the participating sites dictates the availablecooperation solutions. With an array antenna, beam-forming solutions arepossible, while for sites with MIMO antenna setup, cooperation solutionsneed to extend the LTE-A precoding schemes to multi-site scenarios.Moreover, it is quite possible that for NB-relay and NB-home NBcooperation, the cooperating sites have different antenna setup.

Data and CSI sharing possibility allows for more advanced cooperationtechniques. In the NB-NB cooperation, the backhaul latency may limit thecooperation, while in the relay cooperation scenarios, the dominantfactor in data and CSIT sharing is overhead.

RS overhead and channel estimation complexity is another aspect tostudy. While superposition dedicated RS for some solutions maintain theRS overhead and complexity, some solutions require separate channelestimation from different sites for demodulation and/or precoderselection.

3 Cooperation Solutions

Based on the attributes of the cooperating sites, different multi-sitecooperation levels are possible. Based on the CSI knowledge at thetransmitter, we can generalize the multi-site solutions into three maincategories. Open loop, closed loop and semi closed cooperationtechniques.

3.1 Open Loop Cooperation Schemes

An open loop cooperation scheme use all the antenna ports at thecooperating sites to maximize the transmit diversity or throughput ofthe user. In OL cooperation, none of the cooperating sites have accessto channel state information and thus, rely on multi-site and/ortransmit diversity. For high geometry users, different sites transmitindependent data streams to enhance the user experience. Thesetechniques need independent channel estimation from all cooperatingsites. Moreover, for the transmit diversity solutions, full data sharingis required.

3.1.1 Band Switching Transmit Diversity

To improve the coverage to the cell edge users, the two (or more) sitesspecify different bands to the user. The other sites either keep quietin the specified bands from other sites or send low power data to theirown cell centre users. Within the sub-band allocated to each user, asingle-site open loop scheme is utilized. The main difference betweenthis technique and FFR is that this technique is enabled throughscheduling. Also, to achieve multi-site diversity, the transmitted datafrom all the sites should come from the same codebook. Without datasharing, there is no multi-site diversity gain and the only remaininggain is interference avoidance.

Band switching transmit diversity is robust against small timing andfrequency mismatch between the cooperating sites. However, it achievesthe least possible diversity gain.

3.1.2 Tone Switching Transmit Diversity

This technique is similar to band switching transmit diversity exceptthat the tones from different sites are interlaced along time orfrequency direction. Therefore, it achieves higher frequency diversitythan the former one. However, it makes it more susceptible tosynchronization mismatch between the two sites. Similar to othermulti-site TxD schemes, the UE should estimate the channel from all thesites. However, this method induces a coloured noise to neighbouring(non-cooperating) cells.

3.1.3 Space-Time/Frequency Transmit Diversity

Similar to single-site transmit diversity, space-time/frequency blockcodes can achieve high diversity order for all turbo coding rates.However, the total number of antennas in the code increases with theincrease in the number of cooperating sites. Hence, bigger S-T/F codesare required. One solution is to reuse the existing transmit diversityschemes and combine single-site S-T/F codes with tone switching similarto the TxD scheme in LTE 4-Tx transmit diversity scheme. Space-tonecooperation between the sites maintains the spectral density of theinterference to the neighbouring cells.

3.1.4 Multi-Site Spatial Multiplexing

For UEs with high geometry from more than one site, spatial multiplexingimproves the user throughput and also total sector throughput. Withmulti-site spatial multiplexing, each site sends its own data and thereis no need to exchange data between the sites. Moreover, by exploiting aSIC receiver, the total throughput can be further improved. Similar tomulti-site TxD schemes, (ignoring the frequency selectivity of channel)the interference to other sites remain white.

3.2 Closed Loop Cooperation Schemes

With access to the channel state information, closed loop cooperation isavailable between the sites. Depending on the CSI knowledge type, itsaccuracy and how much this information is shared between the sites,different cooperation solutions are possible. For TDD systems, theuplink sounding channel provides access to the DL channel coefficients.For FFD systems, this information is collected by the user feedback oruplink AoA in array sites. Although for the array sites, thebeam-forming matrix does not change fast even for moderate and highspeed users, closed loop cooperation between the sites is sensitive toUE movement and hence is limited to fixed and low speed UEs.

3.2.1 Multi-Site Beam-Forming

Sites with array antennas may use the uplink AoA information for closedloop operation. When two array sites cooperate to send the same data tothe UE using beam-forming, a mechanism to ensure constructive additionof the two beams is required. For this purpose, a timing/distanceadjustment as well as phase correction is required. For demodulationpurposes, the two sites can apply superposition dedicated RS to maintainRS overhead and simple decoding.

3.2.1.1 Timing/Distance Adjustment

Timing mismatch between the cooperating sites as well as differentdistance to the UE results in a mismatch between the arrival times ofthe signal from different times. This timing mismatch results in alinear phase over frequency. A mechanism to estimate the timing errorand correcting it is required.

3.2.1.2 Phase Adjustment

After correcting the linear phase between the two beams, the residualphase difference between the two sites needs to be corrected. Unlike thebeam-forming precoder which is constant over frequency for each site,the phase difference may change over the band due to residual timingmismatch and channel dispersion. The UE may take one site as thereference and report the phase differences to all other sites.

3.2.2 Multi-Site Closed Loop Precoding

When two or more MIMO sites are cooperating, each site applies precodersto the transmitted signal. Similar to multi-site BF, the goal is to makethe signal from all participating sites add constructively at thereceiver. Multi-site precoding is less sensitive to timing/distancemismatch compared to the multi-site BF because of the frequencyselectivity nature of the precoder. Still, timing adjustment shouldensure a relatively constant phase from all sites over the precodingreport sub-band size.

3.2.2.1 Individual Precoding Report

The UE may report individual precoding matrices to different sites. Thisway, the codebook from single-site closed loop is reused. Also, thecodeword selection criteria remain the same. However, a phase adjustmentbetween different sites is required similar to multi-site beam-forming.

3.2.2.2 Aggregate Precoding Report

Here, the UE assumes that all the antennas from all the ports are fromthe same site and find a precoder that best matches the entire antennaset. The UE finds the precoding matrix using a bigger precoder codebook.Each site uses a portion of the precoding matrix corresponding to itsantenna ports for transmitting data to the user. By using only one PMI,there is no need for phase adjustment between the sites.

3.2.3 Closed Loop Cooperation Between Array and MIMO Sites

The aforementioned techniques for multi-site cooperation can be extendedto cooperation between array and MIMO sites.

3.3 Semi Closed Loop Cooperation Techniques

As mentioned before, closed loop cooperation techniques are sensitive toUE movement, timing and phase mismatches. They also require highercomplexity and feedback overhead compared to single-site closed loopschemes. Open loop cooperation between sites each performing a closedloop transmission to the UE is a reasonable compromise that maintainsthe feedback overhead and complexity while benefiting from multi-sitediversity and closed loop gain. While semi closed loop techniques do notachieve the full cooperation gain, they offer the following advantages.

-   -   Easier implementation by reusing single site feedback signaling        and closed loop techniques    -   No need for beam phase correction    -   No need for fine timing/distance adjustment    -   Facilitate cooperation between MIMO and array sites    -   More robust against channel aging        -   Channel coefficients from the same site age in the same way            especially with LoS or array antennas    -   More robust against carrier frequency synchronization errors

3.3.1 Multi-Site Beam-Forming Transmit Diversity

Two or more array sites can cooperate to use a transmit diversity scheme(like the Alamouti code) to send the same data stream to the UE. Acoarse timing adjustment is enough for beam-forming transmit diversityand no phase correction is required. The drawback of this method is thatthe UE needs orthogonal dedicated RS from different sites as independentchannel estimation from different sites is needed.

3.3.2 Multi-Site Closed Loop Transmit Diversity

Similar to multi-site beam-forming transmit diversity, two or more MIMOsites use a space-time code to transmit data to the UE. Again, thesensitivity to timing errors is very low and there is no need for phaseadjustment. The system can reuse the single-site closed loop methods.

3.3.3 Multi-Site Closed-Loop/Beam-Forming SM

Similar to open loop multi-site spatial multiplexing, for high geometryUEs, the cooperating sites send independent data streams to the UE. TheUE reports individual precoders to the cooperating sites in the MIMOcase. For array antenna setup, the UL AoA information is used for BFpurposes. The precoder selection criteria can include minimizinginter-layer interference between different sires.

3.4 Multi-Site Multi-User Cooperating

Multi-site single user cooperation improves user throughput and coverageat the expense of lower frequency reuse factor. If two (or more) UEs arein the coverage area of the same two (or more) sites, multi-sitemulti-user cooperation can improve the user experience while benefitingfrom multi-user techniques to improve total cell throughput.

Interference alignment technique (also called as X-MIMO) can reduce theinterference dimension at the users and hence, increase the total numberof layers transmitted to the users [5].

4 Summary

In this contribution, we provided some study points for the cooperationscenarios between different sites and provided some solutions forfurther study to be adopted by the LTE-A standard. We studied thecooperation in three categories: open loop, closed loop and semi-closedloop. Backhaul overhead to share data and CSI, RS overhead, feedbackoverhead, complexity and sensitivity to timing error, distance and phasemismatch are among parameters that need to be addressed for differentcooperation solutions.

Table 1 provides some details on the requirements on differentalgorithms and their expected gain.

TABLE 1 Cooperation solutions and requirements RS for Demodulationprecoder Data CSI at Scheme RS selection Exchange transmitter Antennasetup Note on gain Band switching Orthogonal NA None or None BothInterference transmit full for avoidance + diversity Multi-sitefrequency diversity selective scheduling Tone switching Orthogonal NANone or None Both Interference transmit full for avoidance + diversityMulti-site frequency diversity diversity Space/Tone Orthogonal NA FullNone Both Spatial transmit diversity diversity OL SM Orthogonal NA NoneNone Both High throughput + SIC Multi-site BF Superposition NA FullAoA + phase Array BF gain dedicated correction Multi-site CLSuperposition orthogonal Full Individual MIMO CL gain (individualdedicated or common precoder Precoder) orthogonal report + common phasecorrection + timing adjustment Multi-site CL Superposition orthogonalFull Collective MIMO CL gain (May (aggregate dedicated or commonprecoder + need bigger Precoder) orthogonal timing precoder set) commonadjustment Heterogeneous Superposition orthogonal Full AoA (array) +Heterogeneous BF and CL multi-site dedicated common precoder gain CL/BF(MIMO) + phase correction + timing adjustment Multi-site CL orthogonalorthogonal Full Individual MIMO Multi-site TxD dedicated or commonprecoder diversity + CL orthogonal (maximize gain common per site power)Multi-site CL orthogonal NA Full AoA Array Multi-site TxD dedicateddiversity + BF gain Multi-site orthogonal orthogonal Full IndividualBoth Multi-site Heterogeneous dedicated or common precoder + diversity +CL TxD orthogonal AoA gain + BF common gain Multi-site BF Orthogonal NANone AoA Array High SM dedicated throughput + SIC Multi-site closedorthogonal orthogonal None Individual MIMO High loop SM dedicated orcommon precoder Throughput + orthogonal (minimize SIC commoninterference) Heterogeneous Orthogonal orthogonal None AoA (array) +Heterogeneous High multi-site SM dedicated or common precoderthroughput + common (MIMO) SIC X-MIMO Orthogonal orthogonal PartialChannel MIMO Interference dedicated or common coefficients alignment +orthogonal high common throughput + SIC

REFERENCES

-   -   [1] Alcatel Shanghai Bell, Alcatel-Lucent, “DL Collaborative        MIMO for LTE-A,” R1-082812, 3GPP TSG RANI #54, Jeju, Korea, Aug.        18-22, 2008.    -   [2] Samsung, “Inter-Cell Interference Mitigation through Limited        Coordination,” R1-082886, 3GPP TSG RANI #54, Jeju, Korea, Aug.        18-22, 2008.    -   [3] Ericsson, “LTE-Advanced—Coordinated Multipoint        transmission/reception,” R1-083069, 3GPP TSG RANI #54, Jeju,        Korea, Aug. 18-22, 2008.    -   [4] LG Electronics, “Network MIMO in LTE-Advanced,” R1-082942,        3GPP TSG RANI #54, Jeju, Korea, Aug. 18-22, 2008.    -   [5] M. A. Maddah-Ali, A. S. Motahari, A. K. Khandani,        “Communication over MIMO X Channels: Interference Alignment,        Decomposition, and Performance Analysis,” IEEE Trans. on        Information Theory, Volume 54, August 2008, pp. 3457-3470.

Wireless Systems with Multi-Transmitters and Multi-Receivers Background

-   -   Conventionally, in wireless systems, one of the following        configurations has been employed:        -   Some transmitters send data to only one of the receiver            (e.g. Uplink Channel, Multi-access channel)—See FIG. 11.        -   Some receivers receive data only from one transmitter (e.g.            Downlink Channel, Broadcast Channel)—See FIG. 12.        -   Each receiver receives data from one of the intended            transmitters (e.g. Interference Channels)—See FIG. 13.

Proposed Scheme

-   -   Here, we propose a new scenario of communication in which in a        system with multiple transmitters and receivers (see FIG. 14,        wherein in ΔT₁ time slot and in ΔF₁ bandwidth, we have one        configuration, while in ΔT₂ time slot and in ΔF₂ bandwidth, we        have another configuration)        -   Each transmitter transmits data to several receivers,        -   Each receiver receives data from several transmitters,        -   The transmission can be done at the same time slot and same            frequency bandwidth        -   In each bandwidth and time slot, the configuration of            communication may be different from the configuration of the            other bandwidth and time slot.        -   Signals transmitted in different time and different            frequencies can be dependent or independent    -   Example: In the system shown in FIG. 14, we have,        -   6 nodes        -   In ΔT₁ time slot and in ΔF₁ bandwidth,            -   Node 1 sends data to nodes 3 and 4            -   Node 2 sends data to nodes 3, 4, 5 and 6            -   Node 3 receives data from nodes 1 and 2            -   Node 4 receives data from nodes 1 and 2            -   Nodes 5 and 6 receive data only from node 2            -   Signal transmitted by nodes one and two can be dependent                or independent        -   In ΔT₂ time slot and in ΔF₂ bandwidth,            -   Node 1 sends data to nodes 3 and 5            -   Node 2 sends data to nodes 3 and 5.            -   Node 6 sends data to nodes 3, 4, and 5.            -   Node 3 receives data from nodes 1, 2, and 6.            -   Node 4 receives data from nodes 1 and 6.            -   Nodes 5 receives data from nodes 1, 2, and 6.            -   Signal of the nodes 1, 2, and 6 can be dependent or                independent        -   Signals of the nodes 1 and 2 in ΔT₁ time slot and in ΔF₁            bandwidth and signals of the nodes 1, 2, and 6 in ΔT₂ time            slot and in ΔF₂ bandwidth can be dependent or independent.

EXAMPLE Multi-Antenna Systems

As an example of the proposed scheme, the following scenario has beendetailed:

Multiple-Antenna System with Two Transmitters and Two Receivers:

We consider a MIMO system with two transmitters and two receivers, where

-   -   Transmitter t, t=1; 2, is equipped with m, antennas    -   Receiver r, r=1, 2, is equipped with n, antennas.    -   The channel between transmitter t and receiver r is represented        by the channel matrix H_(rt), where H_(rt) is a n_(r) by m_(t)        matrix. The received vector y_(r) by receiver r, r=1; 2, is        given by,

y ₁ =H ₁₁ s ₁ +H ₁₂ s ₂ +w ₁   (Eq 1)

y ₂ =H ₂₁ s ₁ +H ₂₂ s ₂ +w ₂   (Eq 2)

-   -   where        -   s_(t) represents the transmitted vector by transmitter t        -   w_(r) is noise vector at receiver r        -   y_(r) is the received vector at receiver r    -   In this system (see FIG. 15),        -   Transmitter 1 sends b₁₁ data streams to receiver one and b₂₁            data streams to receiver two        -   Transmitter 2 sends b₁₂ data streams to receiver one and b₂₂            data streams to receiver two.        -   Transmitters one and two cooperate to send b_(1c) data            streams to receiver one.        -   Transmitters one and two cooperate to send b_(2c) data            streams to receiver two.        -   The six sets of data streams can be dependent or            independent.

EXAMPLE ZF Scheme

-   -   This scheme can be utilized in many applications        -   Example: Using multiple relays in downlink (see FIG. 16)        -   Example: Using multiple relays in uplink (see FIG. 17)        -   Example: Using multiple relays for interference links (See            FIG. 18)    -   To modulate or demodulate the data streams, any linear or        non-linear scheme can be applied.    -   Numbers b_(rt) and b_(rc) can be selected based on design        requirements.    -   As an example, we investigate a scheme based on        -   ZF linear pre-preprocessing and post-processing such that            the data streams have no interference over each other    -   In this example, we assume that n₁=n₂=m₁=m₂=m.    -   In this example (See FIG. 19)

s ₁ =V ₁₁ d ₁₁ +V ₁₂ d ₁₂ +V _(1c) _(—) ₁ d _(1c) +V _(2c) _(—) ₁ d_(2c)   (Eq 3)

s ₂ =V ₁₂ d ₁₂ +V ₂₂ d ₂₂ +V _(1c) _(—) ₂ d _(1c) +V _(2c) _(—) ₂ d_(2c)   (Eq 4)

-   -   Where        -   d_(rt) is a b_(rt) dimensional vector, r, t=1,2, which            include b_(rt) data streams        -   d_(1c) is a b_(1c) dimensional vector, r=1,2, which include            b_(1c) data streams        -   d_(2c) is a b_(2c) dimensional vector, r=1,2, which include            b_(2c) data streams        -   V_(rt) is a m times b_(rt) matrix, r,t=1,2 which include            b_(rt) data stream        -   V_(1c) _(—) ₁ and V_(1c) _(—) ₂ are m times b_(1c) matrices        -   V_(2c) _(—) ₁ and V_(2c) _(—) ₂ are m times b_(2c) matrices        -   To decode d_(rt), the received vector y_(r) is passed            through a filter U_(rt)Q_(r)        -   To decode d_(1c), the received vector y₁ is passed through a            filter U_(1c)Q₁        -   To decode d_(2c), the received vector y₂ is passed through a            filter U_(2c)Q₂

In what follows, the design steps to select system parameter isexplained.

Design Steps:

-   -   Step 1: Choosing integers b_(rt), r,t=1,2 and b_(rc), r=1,2        -   Select integers b_(rt), r,t=1,2 and b_(rc), r=1,2, such that            the following constraints satisfy:

b1c: b _(1c) +b _(2c) +b ₂₂ +b ₂₁<=2m   (Eq 5)

b2c: b _(1c)+b_(2c) +b ₁₁ +b ₁₂<=2m   (Eq 6)

b11: b ₁₁ +b _(2c) +b ₂₂ +b ₂₁ <=m   (Eq 7)

b12: b ₁₂ +b _(2c) +b ₂₂ +b ₂₁ <=m   (Eq 8)

b21: b ₂₁ +b _(1c) +b ₁₁ +b ₁₂ <=m   (Eq 9)

b22: b ₂₂ +b _(1c) +b ₁₁ +b ₁₂ <=m   (Eq 10)

b ₁₁ +b ₂₁ +b _(1c) <=m   (Eq 11)

b ₁₁ +b ₂₁ +b _(2c) <=m   (Eq 12)

b ₁₂ +b ₂₂ +b _(1c) <=m   (Eq 13)

b ₁₂ +b ₂₂ +b _(1c) <=m   (Eq 14)

b ₁₁ +b ₁₂ +b ₂₁ +b ₂₂ +b _(1c) +b _(2c)<=2m   (Eq 15)

-   -   -   Remark: Each of the first four inequalities corresponds to            one of the parameters b_(rt), b_(rc), r, t=1, 2, in the            sense that if b_(rt), or b_(rc) r, t=1;2, is zero, the            corresponding inequality is removed from the set of            constraints.        -   Remark: Based on the application, some new constraint may be            added to the system        -   Remark: If in an application, we are not interested in            common messages, we can choose b_(1c) and b_(2c) as zero.

    -   Step 2: Choosing matrices Q₁ and Q₂        -   Choose matrix Q₁ as an (b_(1c)+b₁₁+b₁₂) times m arbitrary            matrix. Similarly, choose matrix Q₂ as an (b_(2c)+b₂₁+b₂₂)            times m arbitrary matrix.        -   Remark: Q₁ and Q₂ can be chosen based on any optimization            criteria.

    -   Step 3: Choosing modulation matrices:        -   Select modulation matrix V₁₁ such that columns of V₁₁ span            null spaces of Q₂H₂₁.        -   Select Modulation matrix V₂₁ such that columns of V₂₁ span            null spaces of Q₁H₁₁.        -   Select modulation matrix V₁₂ such that columns of V₁₂ span            null spaces of Q₂H₂₂.        -   Select modulation matrix V₂₂ such that columns of V₂₂ span            null spaces of Q₁H₁₂.        -   Select modulation matrices V_(1c) _(—) ₁ and V_(1c) _(—) ₂            such that columns of [(V_(1c) _(—) ₁)^(T), (V_(1c) _(—)            ₂)^(T)]^(T) span null space of the [(Q₂H₂₁)^(T),            (Q₂H₂₂)^(T)]^(T.)        -   Select modulation matrices V_(2c) _(—) ₁ and V_(2c) _(—) ₂            such that columns of [(V_(2c) _(—) ₁)^(T),(V_(2c) _(—)            ₂)^(T)]^(T) span null space of the            [(Q₁H₁₂)^(T),(Q₁H₁₁)^(T)]^(T.)

    -   Step 4: Choosing demodulation matrices:        -   U₁₁ is selected such that the columns of U₁₁ is orthogonal            to the columns of Q₁H₁₂V₁₂ and Q₁[H₁₁H₁₂] [(V_(1c) _(—)            ₁)^(T),(V_(1c) _(—) ₂)^(T)]^(T).        -   U₁₂ is selected such that the columns of U₁₂ is orthogonal            to the columns of Q₁H₁₁V₁₁ and Q₁[H₁₁H₁₂] [(V_(1c) _(—)            ₁)^(T),(V_(1c) _(—) ₂)^(T)]^(T).        -   U_(1c) is selected such that the columns of U_(1c) is            orthogonal to the columns of Q₁H₁₁V₁₁ Q₁H₁₂V₁₂.        -   U₂₁ is selected such that the columns of U₂₁ is orthogonal            to the columns of Q₂H₂₂V₂₂ and Q₂[H₂₁H₂₂] [(V_(2c) _(—)            ₁)^(T),(V_(2c) _(—) ₂)^(T)]^(T).        -   U₂₂ is selected such that the columns of U₂₂ is orthogonal            to the columns of Q₂H₂₁V₂₁ and Q₂[H₂₁H₂₂] [(V_(2c) _(—)            ₁)^(T),(V_(2c) _(—) ₂)^(T)]^(T).        -   U_(2c) is selected such that the columns of U_(2c) is            orthogonal to the columns of Q₂H₂₁V₂₁ and Q₂H₂₂V₂₂.

    -   Remark: Equations (Eq 5) to (Eq 15) guarantee that we can design        such transmit and receive filters.

    -   Remark: The above steps are based on nulling the interference of        data streams over each other. Other linear or nonlinear schemes        like MMSE scheme, successive decoding, dirty-paper-coding, etc.        can be used instead of zero-forcing filters.

EXAMPLE ZF Scheme with Frequency Extension

In the above scheme, we assume that each node has m antennas, providingm space dimensions. Apparently, it is possible to provide dimensionsusing time and frequency resources. In what follows, as an example, weextend the above example to the case, where J frequency sub-bands arealso available.

Multiple-Antenna System with Two Transmitters and Two Receivers and JSub-Bands

Transmitter t, t=1; 2, is equipped with m_(t) antennas

Receiver r, r=1, 2, is equipped with n_(r) antennas.

The channel between transmitter t and receiver r is at sub-band j, j=1,. . . , J, represented by the channel matrix H_(rt)(j), where H_(rt)(j)is a n_(r) by m_(t) complex matrix. The received vector y_(r)(j) byreceiver r, r=1; 2, is given by,

y ₁(j)=H ₁₁(j)s ₁(j)+H ₁₂(j)s ₂(j)+w _(i)(j)   (Eq 16)

y ₂(j)=H ₂₁(j)s ₁(j)+H ₂₂(j)s ₂(j)+w ₂(j)   (Eq 17)

where

s_(t)(j) represents the transmitted vector by transmitter t at frequencysub-band j

w_(r)(j) is noise vector at receiver r at frequency sub-band j

y_(r)(j) is the received vector at receiver r at frequency sub-band j

We define H_(rt), s_(r), and y_(r) as follows:

${H_{rt} = \begin{bmatrix}{H_{rt}(1)} & 0 & 0 & 0 \\0 & {H_{rt}(2)} & 0 & 0 \\0 & 0 & \ddots & 0 \\0 & 0 & 0 & {H_{rt}(J)}\end{bmatrix}},{s_{t} = {{\begin{bmatrix}{s_{t}(1)} \\{s_{t}(1)} \\\vdots \\{s_{t}(J)}\end{bmatrix}\mspace{11mu} y_{r}} = {\begin{bmatrix}{y_{r}(1)} \\{y_{r}(1)} \\\vdots \\{y_{r}(J)}\end{bmatrix}r}}},{t = 1},2$

-   -   As an example, here again we use ZF filter design    -   In this example we assume that n₁=n₂=m₁=m2=m.    -   In this example,

s ₁ =V ₁₁ d ₁₁ +V ₁₂ d ₁₂ +V _(1c) _(—) ₁ d _(1c) +V _(2c) _(—) ₁ d_(2c)   (Eq 18)

s ₂ =V ₁₂ d ₁₂ +V ₂₂ d ₂₂ +V _(1c) _(—) ₂ d _(1c) +V _(2c) _(—) ₂ d_(2c)   (Eq 19)

-   -   Where        -   d_(rt) is a b_(rt) dimensional vector, r,t=1,2, which            include b_(rt) data streams        -   d_(1c) is a b_(1c) dimensional vector, r=1,2, which include            b_(1c) data streams        -   d_(2c) is a b_(2c) dimensional vector, r=1,2, which include            b_(2c) data streams        -   V_(rt) is a m times b_(rt) matrix, r,t=1,2 which include            b_(rt) data stream        -   V_(1c) _(—) ₁ and V_(1c) _(—) ₂ are J.m times b_(1c)            matrices        -   V_(2c) _(—) ₁ and V_(2c) _(—) ₂ are J.m times b_(2c)            matrices        -   To decode d_(rt), the received vector y_(r) is passed            through a filter U_(rt)Q_(r)        -   To decode d_(1c), the received vector y₁ is passed through a            filter U_(1c)Q₁        -   To decode d_(2c), the received vector y₂ is passed through a            filter U_(2c)Q₂

Design Steps:

-   -   Step 1: Choosing integers brt, r,t=1,2 and brc, r=1,2        -   Select integers brt, r,t=1,2 and brc, r=1,2, such that the            following constraints satisfy:

b1c: b _(1c) +b _(2c) +b ₂₂ +b ₂₁<=2J.m   (Eq 20)

b2c: b _(1c) +b _(2c) +b ₁₁ +b ₁₂<=2J.m   (Eq 21)

b11: b ₁₁ +b _(2c) +b ₂₂ +b ₂₁ <=J.m   (Eq 22)

b12: b ₁₂ +b _(2c) +b ₂₂ +b ₂₁ <=J.m   (Eq 23)

b21: b ₂₁ +b _(1c) +b ₁₁ +b ₁₂ <=J.m   (Eq 24)

b22: b ₂₂ +b _(1c) +b ₁₁ +b ₁₂ <=J.m   (Eq 25)

b ₁₁ +b ₂₁ +b _(1c) <=J.m   (Eq 26)

b ₁₁ +b ₂₁ +b _(2c) <=J.m   (Eq 27)

b ₁₂ +b ₂₂ +b _(1c) <=J.m   (Eq 28)

b ₁₂ +b ₂₂ +b _(1c) <=J.m   (Eq 29)

b ₁₁ +b ₁₂ +b ₂₁ +b ₂₂ +b _(1c) +b _(2c)<=2J.m   (Eq 30)

-   -   -   Remark: Each of the first four inequalities corresponds to            one of the parameters b_(rt), b_(rc), r, t=1, 2 in the sense            that if b_(rt), or b_(rc) r, t=1;2, is zero, the            corresponding inequality is removed from the set of            constraints.        -   Remark: Based on the application of the proposed scheme,            some new constraint may be added to the system        -   Remark: If based on the application, we are not interested            in common messages, we can choose b_(1c) and b_(2c) as zero.

    -   Step 2: Choosing matrices Q₁ and Q₂        -   choose matrix Q₁, as an (b_(1c)+b₁₁+b₁₂) times m arbitrary            matrix. Similarly, choose matrix Q₂ as an (b_(2c)+b₂₁+b₂₂)            times m arbitrary matrix.        -   Remark: Q₁ and Q₂ can be chosen based on any optimizing            criteria

    -   Step 3: Choosing modulation matrices:        -   Select modulation matrix V₁₁ such that columns of V₁₁ span            null spaces of Q₂H₂₁.        -   Select Modulation matrix V₂₁ such that columns of V₂₁ span            null spaces of Q₁H₁₁.        -   Select modulation matrix V₁₂ such that columns of V₁₂ span            null spaces of Q₂H₂₂.        -   Select modulation matrix V₂₂ such that columns of V₂₂ span            null spaces of Q₁H₁₂.        -   Select modulation matrices V_(1c) _(—) ₁ and V_(1c) _(—) ₂            such that columns of [(V_(1c) _(—) ₁)^(T), (V_(1c) _(—)            ₂)^(T)]^(T) span null space of the [(Q₂H₂₁)^(T),            (Q₂H₂₂)^(T)]^(T.)        -   Select modulation matrices V_(2c) _(—) ₁ and V_(2c) _(—) ₂            such that columns of [(V_(2c) _(—) ₁)^(T), (V_(2c) _(—)            ₂)^(T)]^(T) span null space of the [(Q₁H₁₂)^(T),            (Q₁H₁₁)^(T)]^(T.)

    -   Step 4: Choosing demodulation matrices:        -   U₁₁ is selected such that the columns of U₁₁ is orthogonal            to the columns of Q₁H₁₂V₁₂ and Q₁[H₁₁H₁₂][(V_(1c) _(—)            ₁)^(T),(V_(1c) _(—) ₂)^(T)]^(T).        -   U₁₂ is selected such that the columns of U₁₂ is orthogonal            to the columns of Q₁H₁₁V₁₁ and Q₁[H₁₁H₁₂][(V_(1c) _(—)            ₁)^(T),(V_(1c) _(—) ₂)^(T)]^(T).        -   U_(1c) is selected such that the columns of U_(1c) is            orthogonal to the columns of Q₁H₁₁V₁₁ and Q₁H₁₂V₁₂.        -   U₂₁ is selected such that the columns of U₂₁ is orthogonal            to the columns of Q₂H₂₂V₂₂ and Q₂[H₂₁H₂₂][(V_(2c) _(—)            ₁)^(T),(V_(2c) _(—) ₂)^(T)]^(T).        -   U₂₂ is selected such that the columns of U₂₂ is orthogonal            to the columns of Q₂H₂₁V₂₁ and Q₂[H₂₁H₂₂][(V_(2c) _(—)            ₁)^(T),(V_(2c) _(—) ₂)^(T)]^(T).        -   U_(2c) is selected such that the columns of U_(2c) is            orthogonal to the columns of Q₂H₂₁V₂₁ and Q₂H₂₂V₂₂.        -   Remark: Remark: (Eq 20) to (Eq 30) guarantee that we can            design such transmit and receive filters.

Advantage of the Proposed Scheme

To show the advantage of the proposed scenario, we consider a downlinksystem with

-   -   One base station with 8 antennas (or 8 available dimensions)    -   Two relays each with 4 antennas    -   To receiver each with 4 antennas

Consider a period of time T.

In what follows, we evaluate three signaling schemes and compare theoverall rate achieved by each schemes.

Scheme One: Conventional Scheme

-   -   In time period [0, T/3], base station simultaneously sends        -   4 data streams, intended for receiver one, to relay one        -   4 data streams, intended for receiver two, to relay two    -   In time period [T/3,2T/3], relay one sends 4 data streams to        receiver one.    -   In time period [2T/3,T], relay two sends 4 data streams to        receiver two.

The overall throughput of this scheme is 8/3 log(P_(T)), where P_(T)represents total power.

Remark: This is the best achievable rate with conventional scheme.

Remark: The overall incoming data streams by each relay is the same asthe overall outgoing data streams.

Scheme Two: Proposed Scheme where Signals of the Relays are Correlated

-   -   In time period [0,T/2], base station simultaneously sends        -   One data stream, intended to receiver one, to relay one,        -   One data stream, intended to receiver one, to relay two,        -   One data stream, intended to receiver two, to relay one,        -   One data stream, intended to receiver two, to relay two,        -   One data stream, intended to receiver one, to both relays,        -   One data stream, intended to receiver two, to both relays,    -   In time period [T/2, T], relays one and two, send data        simultaneously to receiver one and two, based on proposed scheme        with b₁₁=b₁₂=b₂₁=b₂₂=b_(1c)=b_(2c)=1.

The overall throughput of this scheme is 3 log(P_(T)), where P_(T)represents total power.

Remark: The overall incoming data streams by each relay is the same asthe overall outgoing data streams.

Scheme Three: Proposed Scheme where the Signals of the Relays areUncorrelated

-   -   In time period [0,2T/5], base station simultaneously sends        -   Two data streams, intended to receiver one, to relay one,        -   Two data streams, intended to receiver one, to relay two,        -   Two data streams, intended to receiver two, to relay one,        -   Two data streams, intended to receiver two, to relay two,    -   In time period [2T/5,T], relays one and two, send data        simultaneously to receiver one and two, based on proposed scheme        in with b₁₁=b₁₂=b₂₁=b₂₂=4, b_(1c)=b_(2c)=0, and J=3.

The overall throughput of this scheme is 16/5 log(P_(T)), where P_(T)represents total power.

Remark: The overall incoming data streams by each relay is the same asthe overall outgoing data streams.

It is clear from this example that scheme two and three which are basedon the proposed scheme perform better than conventional schemes.

Key Features

The proposed scenario of Communication improves the performance of thecommunication systems in terms of overall throughput, reliability, andcoverage.

To design such system, we can use any linear or non-linear filters basedon the design requirements.

The ZF scheme, presented in detail, can be applied simply to improve theperformance of the communication system.

The ZF scheme, presented here, can be generalized to any number oftransmitters and receivers, to support any number of transmitters andreceivers.

The processing, designed based on ZF scheme, can be employed andredesigned based on other known scheme such as dirty-paper coding,successive decoding, MMSE filters, etc.

The communication schemes shown in FIG. 16 (uplink system with parallelrelays), FIG. 17 (downlink system with parallel relays), and FIG. 18(Interference channel with parallel relays) are helpful in wirelesscommunication systems. This configurations can be generalized to supportany number of transmitters, relays, and receivers. At relay nodes, anyscheme such as decode-and-forward. Amplify-and-forward, etc, can beemployed.

X-MIMO Systems with Multi-Transmitters and Multi-Receivers

Brief Description of the Gist of the Application

-   -   This Application provides a solution for MP-to-MP MIMO systems        to increase the spectral efficiency by coordinating the        interference.    -   The interference arriving at each receive node is coordinated on        the same subspace, so the signal subspace is expanded→higher        number of streams.        -   Method requirements/procedure            -   Each transmitter has the knowledge of its MIMO channel                information.            -   No data-exchanging required between the transmitters,                and no data-exchanging in the receiver side (non                cooperative transmission/reception).

Brief Statement of the Value to Nortel of the Application

-   -   The basic scheme of X-MIMO can be generalized to many wireless        application, such as multi-hop relay, distributed MIMO        networking.    -   Previous Art:        -   Network MIMO: using an additional backbone system to connect            transmitters or receivers which enable us to apply advance            schemes such as dirty paper precoding.            -   Requires data exchange between transmitters.        -   Using more transmit/receive antennas for a given number of            data streams.

X-MIMO Basic Elements

-   -   Each transmitter sends the pilot for each antenna and the pilot        for each transmitter is orthogonal.    -   Each receiver estimates all the incoming MIMO channels        -   It computes the specific receive filters and feeds back the            compound filter and MIMO channel to a specific transmitter.    -   Each transmitter computes the linear pre-coding filter based on        such feedback information from the receiver.    -   Each transmitter sends the pre-coded data.    -   Each receiver demodulates the corresponding data from filtered        receive signal        Identify the Products in which the Application may be used    -   This Application can be used as Nortel-specific proprietary        implementation.        -   Enterprise solution.        -   Wireless backhaul    -   It can be standardized in the next generation broadband wireless        standards.        -   Nomadic multi-user soft handoff        -   BS/Relay Cooperation

List of Key Features

-   -   This method enables the X-MIMO channels to minimize the        dimensions of the interference.    -   It maximizes the number of communication streams over the        channel for a given number of antennas.    -   The processing can be employed and redesigned based on different        schemes such as dirty-paper coding, successive decoding, MMSE        filters, ZF, etc.    -   This method requires no data communication or CSI exchange        between transmitters.    -   The communication schemes such as uplink system with parallel        relays, downlink system with parallel relays and Interference        channel with parallel relays are some examples of the        application of the proposed method in wireless systems and can        be generalized to support any number of transmitters, relays,        and receivers.

Backups

-   -   Background        -   Wireless system configurations.        -   CL MIMO        -   CL Network MIMO    -   X-MIMO        -   Examples of the advantages of the proposed method

Wireless System Configurations

-   -   Conventionally, in wireless systems, one of the following        configurations has been employed:        -   One transmitter sends data to one receiver.        -   Some transmitters send data to only one of the receiver            (e.g. Uplink Channel, Multi-access channel).        -   Some receivers receive data only from one transmitter (e.g.            Downlink Channel, Broadcast Channel).        -   Each receiver receives data from one of the intended            transmitters (e.g. Interference Channels).

CL MIMO

See FIGS. 20 and 21.

-   -   Point-to-point: there is only one transmitter and one receiver.        -   The transmitter selects a precoder based on the channel.        -   Requires channel knowledge at the transmitter.        -   The maximum number of streams is min(nTx,nRx)    -   Point-to-multipoint: there is only one transmitter but a few        receivers.        -   The transmitter selects a precoder based on the compound            channel.            -   The goal is to minimize the interference among                receivers.        -   Requires channel knowledge at the transmitter.        -   The maximum total number of streams is min(nTx,ΣnRx)

CL Network MIMO

-   -   Multiple transmitters and multiple receivers.    -   Transmitters communicate over a backbone and exchange data        and/or CSI    -   The maximum total number of streams is min(ΣnTx,ΣnRx)

X-MIMO

-   -   Here, we propose a new scenario of communication in which in a        system with multiple transmitters and receivers        -   Each transmitter transmits data to several receivers,        -   Each receiver receives data from several transmitters,        -   The transmission can be done at the same time slot and same            frequency bandwidth        -   In each bandwidth and time slot, the configuration of            communication may be different from the configuration of the            other bandwidth and time slot.        -   Signals transmitted in different time and different            frequencies can be dependent or independent

EXAMPLE

-   -   Two transmitters and two receivers close to each other (strong        interference).        -   The total number of layers with the conventional method is            3.            -   2-layers for link 1 and 1-layer for link 2 (or vice                versa)                -   Receiver one cancels one layer and decodes two                    layers.                -   Receiver two cancels two layers and decodes one                    layer.        -   With proposed method and using ZF, the total number of            layers are 4.            -   Each transmitter sends a layer to receiver one and one                layer to receiver 2.            -   Each decoder cancels two layers of coordinated                interference and decodes two layers.                -   The two interferences at each node are coordinated,                    seen as only one interference stream.

ADVANTAGE OF THE PROPOSED SCHEME (EXAMPLE)

-   -   Consider a downlink system with        -   One base station with 8 antennas (or 8 available dimensions)        -   Two relays each with 4 antennas        -   To receiver each with 4 antennas    -   Consider a period of time T    -   In what follows, we evaluate three signaling schemes and compare        the overall rate achieved by each schemes. See FIG. 22.

Scheme One: Conventional Scheme

-   -   In time period [0,T/3], base station simultaneously sends        -   4 data streams, intended for receiver one, to relay one        -   4 data streams, intended for receiver two, to relay two    -   In time period [T/3,2T/3], relay one sends 4 data streams to        receiver one.    -   In time period [2T/3,T], relay two sends 4 data streams to        receiver two.    -   The overall throughput of this scheme is 8/3 log(P_(T)), where        P_(T) represents total power.    -   Remark: This is the best achievable rate with conventional        scheme.    -   Remark: The overall incoming data streams by each relay is the        same as the overall outgoing data streams.        Scheme 2: X-MIMO with where Signals of the Relays are Correlated    -   In time period [0,T/2], base station simultaneously sends        -   One data stream, intended to receiver one, to relay one,        -   One data stream, intended to receiver one, to relay two,        -   One data stream, intended to receiver two, to relay one,        -   One data stream, intended to receiver two, to relay two,        -   One data stream, intended to receiver one, to both relays,        -   One data stream, intended to receiver two, to both relays,    -   In time period [T/2,T], relays one and two, send data        simultaneously to receiver one and two, based on proposed scheme        with b₁₁=b₁₂=b₂₁=b₂₂=b_(1c)=b_(2c)=1.

The overall throughput of this scheme is 3 log(P_(T)), where P_(T)represents total power.

Remark: The overall incoming data streams by each relay is the same asthe overall outgoing data streams.

Scheme 2: X-MIMO with where Signals of the Relays are Uncorrelated

-   -   In time period [0,2T/5], base station simultaneously sends        -   Two data streams, intended to receiver one, to relay one,        -   Two data streams, intended to receiver one, to relay two,        -   Two data streams, intended to receiver two, to relay one,        -   Two data streams, intended to receiver two, to relay two,    -   In time period [2T/5,T], relays one and two, send data        simultaneously to receiver one and two, based on proposed scheme        in with b₁₁=b₁₂=b₂₁=b₂₂=4, b_(1c)=b_(2c)=0, and J=3.

The overall throughput of this scheme is 16/5 log(P_(T)), where P_(T)represents total power.

Remark: The overall incoming data streams by each relay is the same asthe overall outgoing data streams.

It is clear from this example that scheme two and three which are basedon the proposed scheme perform better than conventional schemes.

See FIGS. 23 and 24.

1. A method for receiving a wireless transmission of a plurality of datastreams in a wireless communication system having a plurality of nodes,each node having multiple antennas, the method comprising: receivingfirst and second data streams from respective first and second nodes ata receiver node; causing said receiver node to generate a receive filterfor decoding each of the received data streams; and causing saidreceiver node to transmit receive filter information for each of saidfirst and second data streams, said receive filter informationfacilitating precoding of said first and second data streams forsimultaneous transmission within a common frequency band to saidreceiver node.